Shaon Barua

MgB2 superconducting joints for persistent current operation

High-performance superconducting joints are essential for realizing persistent-mode magnets. Herein, we propose a concept and fabrication of such superconducting joints, which yielded reliable performance in the operating temperature range of 4.2–25 K. MgB2–MgB2 joints in magnets are known to result in deterioration of localized electrical, thermal, and mechanical properties. To overcome these problems, the ends of the two wires are inserted into a pellet press, which is then filled with a mixture of unreacted magnesium and boron powders, followed by heat treatment. The critical current capacity and joint resistance were precisely evaluated by the standard four-probe method in open-circuit and by field-decay measurements in a closed-loop, respectively. These joints demonstrated up to 66% of the current-carrying capacity of unjoined wire at 20 K, 2 T and joint resistance of 1.4 × 10−12 Ω at 4.2 K in self-field.

Significantly enhanced critical current density in nano-MgB2 grains rapidly formed at low temperature with homogeneous carbon doping

High performance MgB2 bulks using carbon-coated amorphous boron as a boron precursor were fabricated by Cu-activated sintering at low temperature (600 °C, below the Mg melting point). Dense nano-MgB2 grains with a high level of homogeneous carbon doping were formed in these MgB2 samples. This type of microstructure can provide a stronger flux pinning force, together with depressed volatility and oxidation of Mg owing to the low-temperature Cu-activated sintering, leading to a significant improvement of critical current density (Jc) in the as-prepared samples. In particular, the value of Jc for the carbon-coated (Mg1.1B2)Cu0.05 sample prepared here is even above 1 × 105 A cm−2 at 20 K, 2 T. The results herein suggest that the combination of low-temperature Cu-activated sintering and employment of carbon-coated amorphous boron as a precursor could be a promising technique for the industrial production of practical MgB2 bulks or wires with excellent Jc, as the carbon-coated amorphous boron powder can be produced commercially at low cost, while the addition of Cu is very convenient and inexpensive.

The roles of CHPD: superior critical current density and n-value obtained in binary in situ MgB2 cables

A binary magnesium diboride (MgB2) cable has been assembled by braiding six Nb/Monel sheathed monofilament strands around a central copper stabilizer for improving the operational environment. The total critical current (Ic) of the braided cable is obtained by multiplying the Ic of six single wires, without any dissipation. In this work, various mechanical deformations, i.e., swaging, two-axial rolling, groove rolling, and cold high-pressure densification (CHPD) at 1.8 GPa have been applied to the 6-stranded cable to obtain additional densification. The highest critical current density at both 4.2 and 20 K has been achieved in this work through the CHPD treated cable due to higher filament mass density. The present results are promising in view of the cable, particularly in power applications at industrial lengths that pave the way to seeking an optimal protocol to meet a practical functionality.

The formation of nano-layered grains and their enhanced superconducting transition temperature in Mg-doped FeSe0.9 bulks

To search a proper dopant to further improve superconductivity in 11 type Fe-based superconductors makes sense to both their superconductivity mechanism and possible technological applications. In present work, Mg doped FeSe polycrystalline bulks were obtained by a two-step solid-state reaction method. Even though there are many MgSe and iron impurities existing in the Mg heavy doped FeSe bulks, they exhibit obviously increased Tc compared to undoped FeSe sample. It was found that Mg addition has little effect on the crystal lattice parameters of superconducting β-FeSe, whereas leads to the formation of nano-layered grain structure consisted of MgSe and β-FeSe with similar X-ray diffraction characteristics. Lots of nano-structural interfaces between FeSe and MgSe formed in this homogenous layered grain structure have significant effect on the superconducting properties and are responsible for the enhancement of Tc, as like the case of FeSe thin film on some specific substrates. Our work not only demonstrates a powerful way for raising Tc in bulk superconductors, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by considering interface effect.

Development of high current capacity mono- and 18-filament in situ MgB2 cables by varying the twist pitch

Undoped and carbon doped magnesium diboride (MgB2) cables have been assembled by braiding six Nb/Monel and Nb/Cu/stainless steel (SS) sheathed mono- and multifilament strands with a central copper stabilizer for improving the operational environment. This paper presents the fabrication and characterization of two types of in situ powder-in-tube processed mono (pure) and multifilament (carbon doped) MgB2 cables with different twist pitch lengths; thereby making them possible candidates for industrial AC applications. Critical current is not influenced by the cabling that results in various twist lengths. The total critical current of the braided cables is obtained by multiplying the critical current of six single wires without any dissipation. The critical current density (Jc) of pure (mono) and carbon doped (18-filament) six stranded cable reached 10000 A/cm2 at 5.5 and 10 T, respectively; without any observable deleterious effect caused by varying the twist pitch. The engineering current density (Je) of both cables reached the same value of 10000 A/cm2 at 3.5 and 6.5 T, respectively. Compared to the literature, this work reports some of the highest Jc and Je values for carbon doped multifilament cables that remain unaffected upon varying the twist pitch. The present results are promising in terms of scaling up these cables to industrial lengths for transformers, fault-current limiters-based applications and paves the way for the development of optimal protocols for practical functionality.